Which type of perm rod has an even diameter throughout its length?

The type of perm rod that has an even diameter throughout its length is the straight rod. Straight rods are designed to create uniform curls, which is beneficial for achieving consistent texture and curl formation in chemical texturizing processes. Their stability and even diameter allow for a more predictable curl pattern, making them ideal for a variety of hair lengths and textures.

In contrast, spiral rods are typically tapered and designed to produce a spiral curl effect, while curved rods create curls that are more crescent-shaped, contributing to a softer, more natural wave. Flexible rods can be bent into various shapes, allowing for creative styling, but they do not have an even diameter along their length either. Thus, the straight rod's uniform structure makes it uniquely suited for consistent results in perming.